A concierge of an apartment in Barcelona named Cesar is on the roof of a building explaining his nature of not being able to find any happiness no matter what he does or what good things happen to him. As he wakes up in Claras apartment and begins his routine of working in the main lobby, Ursula, the daughter of another tenant, receives hush money from Cesar. Cesar visits his mom who is at the hospital and is awfully sick practically unable to speak to his son but listen to him talk about Clara. Veronica who is another tenant of Cesar asks him for a favor to take care of her dogs giving detailed directions on how to feed them. Entrusted to feed them, Cesar breaking her directions and feeds Rocio a slice of pie. Clara entering her apartment opens up letters she has received then tears them and throws them away all while Cesar is hiding under her bed waiting for her to fall asleep in order to use chloroform to keep her asleep so he can maliciously mess with her care product. It is the morning after sneaks out of Clara's apartment with Ursula waiting outside and demands that she will keep her mouth shut if Cesar provides her with an adult movie. Cesar working in the lobby finds Veronica heading out of the apartment and he questions Veronica where Rocio is and she explains that Rocio has horrible diarrhea to something he may have eaten.Ursula sneaking out of school goes to Cesar's apartment and asks for the videotape and tells him that she and Clara talked about Cesar and the price for her keeping her mouth shut will be 100 euros. Cesar has also successfully angered another of the tenants by not taking care of the plants leading them to die. Visiting his mother at the hospital again Cesar says that he is close to wiping Clara's smile off her face as his goal is to make his tenants frustrated. The cleaners of the apartment are informed by Cesar that the office was not cleaned properly which leads to the cleaner's son being in trouble.Cesar once again hiding under Clara's bed uses chloroform on her to keep her in deep sleep and begins messing with more of Clara's items in an attempt to make her upset and plants bug eggs in her apartment. A couple of days pass and Clara's rashes which she was developing due to Cesar messing with her care products have slightly gone away, relieved for a second she finds many cockroaches in her apartment and decides to stay with her mom till Cesar fumigates her apartment. The police get involved to figure out who is harassing Clara and Cesar frames the cleaners' son which results in him getting arrested.Clara returns to the apartment and while Cesar hides under her bed he finds that she brought her boyfriend Marcos as well. Having accidentally dropped some chloroform on his face, Cesar makes a desperate attempt to leave the apartment quietly but realizes he has the wrong keys to the apartment leaving him locked inside. Cesar waking up in Clara's shower begins to worry as Marcos had found Cesar's sports bag with many suspicious items. Marcos catching Cesar trying to sneak out of the apartment he begins explaining his reason for being there. Successfully deceiving them, he leaves the apartment and heads downstairs only to find out he is gonna be vacated. Clara and Marcos about to go on a trip have to return as Clara turns out to be 4 weeks pregnant and Marco is surprised as he always used a condom when they had sex.Marcos knocks at Cesar's place looking for him and asks that he checks Clara's apartment as he states there is a bug problem again. Cesar deceived to check the place out realizes that Marco has caught onto him sneaking into her place and has found what Cesar has been drugging her with. Cesar and Marco begin to fight and Cesar grabs a piece of glass and stabs Marcos's neck killing him. Knowing that Ursula knows Cesar was involved in Marcos's death, Cesar sneaks into her apartment and threatens Ursula to never rat him out to guarantee that Cesar does not go to jail. Clara having moved out of the apartment and having her baby, Cesar mails her a letter hoping that any time she looks at their child she will think of him, and he thanks her for helping him finally be happy.